Bone constraint rotation not working correctly
I may be wrong but I'm not sure about this method, I think it works on the video because his foot is almost aligned to his controller on every axis, that's a solution by the way, but if they are not aligned, it will be ok as long as you rotate the controller on the X axis (because both bones are aligned on this axis), but, as soon as you rotate the controller on its Y axis or Z axis, the foot will go crazy (because you ask it to rotate on its own Y and Z and that's what it gives).

In my opinion the solution is to parent the foot to the controller.This way, the foot follows the controller no matter what. Then, if you want to make sure that the foot bone will stick to the shin bone, you can give it a Copy Location with Space > Pose Space and a Head/Tail value of 1.